The art of glass drawing is presently the most effective mode of producing either continuous, flexible fibers or of producing relatively short segments for later combining and processing into composite products such as fiberoptic screens, faceplates, and image modifiers of various types. Besides being used for drawing of fibers and multi-fiber bundles, drawing techniques of the type to which the invention relates are applied to late-stage processing of the composite products. Such processing includes cross-sectional reduction, either uniform or graduated, the latter technique used to form image expanders and reducers. Such processing also includes various degrees of twisting and other manipulations to form image re-orienting devices such as partial rotators, inverters, etc.
An important goal in this technical field is uniformity of heating and a high degree of temperature control in the critical softened area of the preform or workpiece. Failure of uniformity in heating the work zone is a major cause of product defect and rejection, resulting in waste of expensive materials and production time. This consideration is particularly critical in the case of a product formed from a preform of highly complex cross-sectional character in which large, sometimes sharp, gradients of optical, physical, and thermodynamic properties are likely to be present. The requirement of uniform heating reaches ultimate criticality when the conventional upper limits of heating and drawing speed and of preform and product cross-sectional dimension are reached and exceeded. It has been the unrealized goal of skilled workers in this art to produce uniform heating in the drawing furnace at the moderate temperatures needed for drawing relatively delicate composite products. Among the main reasons for failure to achieve this goal has been the difficulty of achieving uniform radiation of heat from the radiant heating elements at temperatures of around 1100.degree. F. to 1400.degree. F. (600.degree.to 750.degree. C.) Separate radiant elements produce inherently non-uniform heating. Attempts to produce a radiant source continuously surrounding the fusing area or to embed discrete elements in a diffusing matrix have not produced the desired uniformity. Moreover, most composite products do not absorb radiant energy uniformly even if it is introduced uniformly. This compounds the problem of non-uniform radiant sources, and limits the level of uniformity even for an ideal uniform radiant source.
This problem of absorption differential exists in any application where there are different glasses in the same product, the glasses having different infra-red absorbing characteristics (for instance the core relative to the cladding) or any product involving an extremely thick preform or drawn diameter. In the latter case, the rate of heating by absorption at the surface of the working area must be carefully regulated according to the rate of conduction of the heat toward the center of the piece. At locations toward the axis the radiant energy per se fails to penetrate at levels comparable to that at the surface.
A prior method of approximating uniformly radiating elements has been developed which involves turning the preform and the product on their common axis, at a rate sufficient to smooth out the variations in the radiational heating sources. This technique requires complex mechanisms to coordinate the turning of the preform and product, as well as the turning and lateral translation of the take-up reel if the product requires. At best, the technique produces horizontal (stratified) uniformity without producing vertical uniformity and results in hot "rings" instead of hot "spots". Moreover, the technique does not address the problem of non-uniform absorption by a composite product.
The problem of non-uniform absorption is especially acute when the product contains light-absorbing elements such as EMA cladding or fibers which tend to absorb infra-red radiation in disproportion to the remaining materials. Such elements, in a radiant furnace, produce internal anomalies of temperature and viscosity which limit and complicate the choice of drawing speed.
As a secondary consequence to the inability to achieve uniform heating in the drawing process, both the preform size and the reduction ratio in the drawing process are severely limited. The result is that a composite product having very small diameter fiber-optic components must be produced by a many-step process. Typically, the steps include drawing a single fiber, drawing down a multi-fiber bundle, drawing a multi-multi fiber bundle, and fusing a bundle of these latter products into a block. Such many-stage processes consume production time, and each step has its own percentage rejection rate (on the order of 20%).
Thus, the main object of the present invention is to provide a process for acting on the working area of a preform to product highly controlled, uniform heating.
Another object of the invention is to provide a process in which the limits on the size of the preform, the product, and the drawing reduction ratio are greatly extended.
A further object of the invention is to provide a process for drawing glass which allows the elimination of at least one of the successive reduction drawings in certain fiber-optic processes, without loss of quality.
Another object of the invention is to provide a process in which the size of the working zone at which reduction takes place may be chosen and controlled. In another of its aspects, the invention provides and apparatus particularly adapted to assist in carrying out the uniform heating and control of the working zone of a drawable preform.
Further objects of the invention will become apparent as specific embodiments are described.